The invention relates to a self-locking belt roller with a vehicle-sensitive and/or seat belt-sensitive controllable blocking device, the belt roller being provided with a force limiting device for restricted belt withdrawal when a blocking member is engaged, the force limiting device comprising a torsion rod which is connected on the one hand to the belt winding shaft and on the other hand to the blocking member.
A belt roller having the above features is described in connection with a tightening device in DE 43 31 027 A1; however, the tightening device is not necessary for the function of the force limiting device in the known belt roller. In the known belt roller, the force limiting device is constructed as a torsion rod, so that particularly when used in connection with an airbag, the response of the force limiting device allows for an additional, but decelerated forward displacement of the belted occupant. This is achieved in that, in the event of loading, as the belt unwinds from the belt winding shaft the belt loading of the belt winding shaft blocked by the blocking member is reduced by the torsional rod which yields and allows for further rotation of the belt winding shaft.
The disadvantage associated with the known belt roller is that the acting force is increased by the force limiting device as a result of the cold deformation of the material and the decreasing winding diameter, i.e. as a result of the associated lever arm reduction; the associated force-path characteristic curve is progressive. In this context, further tests have shown that it is, for example, desirable for a higher restraining force to be exerted by the force limiting device at the start of an accident, namely so long as the belted person is not in contact with the airbag, and only to allow a reduced restraining force to be exerted via the belt roller once both safety devices are effective.
It is therefore the object of the invention to use suitable measures to better adapt the force-path curve in a belt roller of the generic type to the special features of the deceleration curve of the vehicle or of the vehicle occupants.